1. Field of the Invention
This invention pertains to electro-explosive devices and other electrical components that require glass-to-metal seals. Specifically, this invention pertains to a method of attaining a hermetic seal between glass and stainless steel where both mechanical and chemical bonds are formed in a metal-to-glass-to-metal three-layer arrangement.
2. Description of the Prior Art
A large spectrum of commercial glass-to-metal seals are known. In some cases, an outer metal layer is added to form a metal-to-glass-to-metal sandwich. These methods use various devices to obtain a stable relationship between the metal and glass. Several prior art seals have claimed "matched coefficients" between the glass and metal. The basic metal used in such seals is that known commercially as Kovar. These glass-to-metal seals use a non-stainless steel and a soft glass to provide a hermetic seal. The finished part is frequently plated with gold or other similar metals to provide a reliable electrical path on the outside of the glass. The glass is usually porous and will not withstand severe shock due to thermal or impact conditions.
These previous glass-to-metal seals, including metal-to-glass-to-metal, which are sometimes described as bonded, are actually to press fit. The chemical bond is actually a glass-to-metal oxide bond that is pressed against the metal or metal-to-metal oxide bond. The glass contains stress fractures. No actual bond has been formed between glass and metal. Stainless steel has not been previously used. The closest approach to an actual stainless steel is U.S. Pat. No. 2,394,919 to Kingston for a Metal-Glass Seal and Sealing Alloy. This patent uses a stainless alloy of 38-45% nickel and 3-15% chromium with the rest iron. A chromium oxide layer is created by heating in a moist hydrogen atmosphere to facilitate a vacuum tight seal between the metal and glass. The atmosphere controls the type of oxide formed. A moist hydrogen atmosphere was believed to be necessary to form oxides capable of bonding glass to metal. The Kingston method forms a black film on the metal which is a glue joint bond. A glue joint bond is one where the upper and lower layers of material form chemical bonds with the glue, but not with each other. Within the Kingston patent it is noted that the alloy used is a new alloy specifically created for this type of glass-to-metal seal. This particular prior art patent was designed for use in vacuum tube techniques. U.S. Pat. No. 2,697,309 to Gates for a Method of Making Tubular Glass-to-Metal Seals also refers to using an alloy material of Kingston to take advantage of the oxidation which improves the glass-to-metal seal characteristics.
FIG. 1 shows a cross-sectional view of these prior art glass-to-metal bonds. A glass layer 10 surrounds a metal pin 12. The oxidation of the metal forming pin 12 is itself a chemical bonded layer 16. The oxide forms a chemical bond with glass 10 which is shown by layer 18. The oxide serves as glue with its upper portion forming a chemical bonded layer 18 with glass 10 and its lower portion forming a chemical bonded layer 16 with metal 12. If an additional metal layer is added around glass 10 it must be thick enough to cause compression bonding of glass 10 to pin 12 if the coefficients of expansion of the glass and metal is not exact. An outer metal layer will have the two oxide bonded layers 16 and 18 in the same relative position between the glass-to-metal interface. No actual glass-metal bonds exist.
None of the known prior art forms complex iron-nickel-chromium oxide which forms a true chemical bond between metal and glass. None of the known prior art has the top and bottom surface of a glass layer bonded chemically and mechanically to metal.